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Do headphones run on AC or DC?

post #1 of 51
Thread Starter 

Sorry if this is a dumb question. But this has been bugging me.

 

It is my understanding that speakers run on AC, and the oscillations are what enable them to vibrate and produce sound. Two wires to a speaker, with the current alternating between each wire, and hence neither one is strictly + or - polarity. And supposing that headphones are miniature speakers, it follows that they would run on AC also.

 

However, the typical unbalanced stereo headphones are powered through a TRS plug. So 3 conductors: left channel signal, right channel signal, and a shared ground channel. If this was AC, then half the time we would be getting the L and R channels mixed together from being sent up to the headphones through the same ground wire. In other words, epic crosstalk or even mono sound. But this has not been my experience.

 

Can someone enlighten me, please?

post #2 of 51

Answer withdrawn.


Edited by hodgjy - 7/18/10 at 8:48pm
post #3 of 51

DC: + and - remain constant.  Voltage fluctuation (difference between +/-) carries the sound signal which should be ideally converted directly to speaker motion.

post #4 of 51

People usually use "DC" and "AC" refer to the power supply input type accepted (and typical dynamic headphones aren't powered by an external source).  This is different than the audio signal.  The audio signal definitely changes over time, so it's AC in that sense.

 

Both L and R headphone inputs can be thought of as some changing values--say between -1 and +1 volts--relative to the ground at 0.  The ground can be shared because it's the reference for both channels.  So the L channel information is just the difference between what's in the L wire and the ground wire.  The L headphone driver is connected between the L wire and ground wire, so it transforms that electrical voltage difference into mechanical vibrations corresponding to that channel's signal.  Likewise, the R channel information is the difference between the R wire and the ground wire, etc.  The voltage on the L wire being negative or positive has nothing to do with whatever is on the R wire (ignoring small crosstalk), so the two signals are kept distinct despite both sharing the same ground.

 

edit: and the current goes from the source L output down the L wire through the L headphone driver to the ground wire and back to the source, and also from the source R output down the R wire through the R headphone driver to the ground wire and back to the source.  Maybe there's an intuitive notion that something would go down the L wire to ground and back up the R wire and thus screw up the channel separation, but it's current we're talking about.  What's of interest is the net movement of current.

 

I hope this explanation wasn't too rudimentary or on the flip side, unnecessarily verbose.

 

Btw...Yoga Fire!


Edited by mikeaj - 7/18/10 at 10:55pm
post #5 of 51
Thread Starter 

Thanks for the responses guys. I think I get it now.

 

Headphones run on DC in that the polarity on the wires does not change. And the current is always only running in one direction. Up through the L + R wires and back down through the shared ground. But there is still "alternating" happening, in that the voltage rises and falls rapidly to create vibrations in the headphone drivers.

 

So does the same thing happen in conventional speakers too? (Meaning that I was wrong about them being AC.) I've read some comments about powerful headphone amps being able to drive some speakers directly.

 

 

 

Also:

→  \  ↓  /  ←   +   punch

post #6 of 51

Headphones and speakers run on AC. The +/- sign is there to help preserve correct phase polarity of wires so that you do not invert the signal while making the connection.  The +/- sign for headphones and speakers has very little to do with the +/- sign used for designating DC polarity.

 

Jack  

post #7 of 51

 

 

Speakers do NOT like DC at all,in fact if they get a good dose of it you can say goodby to them.

post #8 of 51
Quote:

In alternating current (AC) the movement of electric charge periodically reverses direction. In direct current (DC), the flow of electric charge is only in one direction.

 

The polarity remains constant in a headphone, thus strictly speaking a headphone runs DC.

However, the definition of DC is often extended to the constant, zero-frequency, or slowly varying local mean value of a voltage or current, if you consider the extended definition, headphones runs neither DC or AC.

You could simply say that headphones run on the waveform of the audio signal, I think that's the most accurate answer.

post #9 of 51

Charge does reverse direction though,so AC seems to fit the bill.

post #10 of 51

 

Originally Posted by khaos974 View Post

 

The polarity remains constant in a headphone, thus strictly speaking a headphone runs DC.

 

No, the polarity changes constantly in the rhythm of the music signal, which gets transformed into forward and backward motion of the membrane. It's clearly alternating current.
.

post #11 of 51

I guess it does, I didn't think it through. Thanks.
 

Quote:
Originally Posted by JaZZ View Post

No, the polarity changes constantly in the rhythm of the music signal, which gets transformed into forward and backward motion of the membrane. It's clearly alternating current..

post #12 of 51
Thread Starter 

Okay, but if headphones are AC, it has to be a different kind of AC than speakers right? The wiring is different. I mean, imagine combining the "ground" wires on conventional speakers. I'm guessing that's a big no. But stereo unbalanced headphones always have the ground wires combined at the TRS plug.

post #13 of 51
Quote:
Originally Posted by Yoga Flame View Post

Okay, but if headphones are AC, it has to be a different kind of AC than speakers right? The wiring is different. I mean, imagine combining the "ground" wires on conventional speakers. I'm guessing that's a big no. But stereo unbalanced headphones always have the ground wires combined at the TRS plug.



The ground on most amps is common.

post #14 of 51

If you send more than 50 mV's of DC to most headphones the coil will burn up (I've done it.)   A simple test all amp builders do when they are done is check to make sure there is no DC on the output with a DMM.   Headphones and speakers run on AC,  but some dc coupled amps have a small amount of dc current (offset),  less is always better. 

post #15 of 51
Thread Starter 

The thing that bugs me about AC on headphones is that it works with 3 wires from the amp to the phones. I can't wrap my head around how it wouldn't result in severe crosstalk if the current can flow in both directions.

 

 

This is how speakers are normally wired:

 

Speakers wired normally

 

And from my understanding of AC, it doesn't matter how you match the red and black dots on each speaker since current flows both ways regardless.

 

 

And this is how they would be wired if they were like headphones:

 

Speakers wired like headphones

 

Does that look wrong to anyone else?

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